GEOLOGIC RECONNAISSANCE OF SOUTHWESTERN LUZON. 85 



LOBOO MOUNTAINS. 



This region was observed from the vicinity of Batangas and from 

 steamers which passed very near to its southern coast. In 1905 Smith 

 made a partial reconnaissance of the Loboo Mountains, his special object 

 being to examine some copper prospects on the Calbasahan River in the 

 northeastern part of the area. He described the mountains as forming 

 a chain approximately parallel with the coast but broken by the Loboo 

 or Rosario River Valley. From a study of the topography of the region, 

 it seems that two ranges may be distinguished extending in a northeast 

 southwest direction (N. 60° E.) approximately parallel with the Rosario 

 River Valley. If either of these interpretations of the trend of the 

 Loboo Mountains is correct, the structure of the region is discordant 

 with that of the eastern and western Cordilleras, and because of this it 

 is discussed here under a separate heading. 



' Diorites. — The only igneous rocks identified by Smith were diorites 

 which he collected near the copper prospects in the northeastern part of 

 the mountains. This locality would be considered as in the southeastern 

 range if two ranges are distinguished. He found the rock to vary from 

 a typical diorite to granitic and gneissic phases, being usually coarse 

 grained. It is cut by quartz veins, more or less mineralized, and showing 

 stains of iron or copper oxide and carbonates. Some molybdenite was 

 found in the. veins. Inasmuch as no important mineral deposits were 

 developed by the prospecting, interest in the district ceased and there 

 has been no occasion to revisit it. 



Andesites. — Beach gravels . and bowlders from near the Pinamucan 

 River were seen at Batangas. They had been brought in bancas and 

 were being crushed for road metal to be used in the building of the 

 road from Batangas to San Juan. These bowlders are contributed to 

 the beach by the Pinaniucan River and the other streams near by, which 

 drain the western end of the northeastern range of the Loboo Mountains. 

 They are andesitic and indicate the existence of extensive areas of andesite 

 in the mountains. 



Tertiary sedimentaries. — In journeying from "Batangas to the Loboo 

 River Valley, Smith found exposures of what he. described as a basal 

 conglomerate succeeded by shaley sandstones and limestones. Fossils 

 collected from this formation were determined by him as being of 

 Miocene age and younger, certain of them having Bornean affinities. 

 Accordingly, the sedimentary formations of the Loboo Mountains may 

 be the equivalent of those in the eastern cordillera. 



Coralline limestones and marls, — Heavy beds of coralline limestone 



